Swelling soils, also known as expansive soils, are ones that swell in volume when subjected to moisture. These swelling soils typically contain clay minerals that attract and absorb water. When water is introduced to expansive soils, the water molecules are pulled into gaps between the soil plates. As more water is absorbed, the plates are forced further apart, leading to an increase in soil pore pressure (Handy, 1995). If this increased pressure exceeds surcharge pressure (including the weight of the overlying pavement) the soil will expand in volume to a point where these pressures are once again in balance. Swelling pressures can be on the order of 100 – 200 kPa (14.5 – 29 psi) and have been measured as high as 1000 kPa (145 psi). Table 1 gives a general idea of the types of expansion that can be expected.

1. C
SWELLING EXPANSION AND
DILATION OF SOIL
PREPARED BY
Ahmed Zaid Saif
facebook.com/ahmedzaidsaif

2. Introduction
• Swelling soils, also known as expansive soils,
are ones that swell in volume when subjected to
moisture. These swelling soils typically contain
clay minerals that attract and absorb water.
When water is introduced to expansive soils, the
water molecules are pulled into gaps between
the soil plates. As more water is absorbed, the
plates are forced further apart, leading to an
increase in soil pore pressure (Handy, 1995). If
this increased pressure exceeds surcharge
pressure (including the weight of the overlying
pavement) the soil will expand in volume to a
point where these pressures are once again in
balance. Swelling pressures can be on the order
of 100 – 200 kPa (14.5 – 29 psi) and have been
measured as high as 1000 kPa (145 psi). Table
1 gives a general idea of the types of expansion
that can be expected.

3. First: Swelling (Expansive) Soils:
• Swelling soil is soil containing montmorillonite clay
minerals. Swelling soil can cause damage when
the soil changes volume, due to the soil either
expanding when moisture is added, or shrinking
when the soil dries out. A soil that has moderate to
high swell potential is capable of causing uplift to
concrete slabs and other property damage.
• Swelling soils, also known as expansive soils, are
ones that swell in volume when subjected to
moisture. These swelling soils typically contain
clay minerals that attract and absorb water. When
water is introduced to expansive soils, the water
molecules are pulled into gaps between the soil
plates. As more water is absorbed, the plates are
forced further apart, leading to an increase in soil
pore pressure (Handy, 1995).

4. Swelling (Expansive) Soils:
• If this increased pressure exceeds surcharge
pressure (including the weight of the overlying
pavement) the soil will expand in volume to a point
where these pressures are once again in balance.
• Swelling pressures can be on the order of 100 –
200 kPa (14.5 – 29 psi) and have been measured
as high as 1000 kPa (145 psi). Table 1 gives a
general idea of the types of expansion that can be
expected.

5. • The magnitude of the volume change and
subsequent damage is influenced by several
factors:
the amount of moisture change;
the amount of expansive clay in the soil;
the soil density;
the amount of structural loading; and
the type of minerals in the clay.
Swelling (Expansive) Soils:

6. Swelling (Expansive) Soils:
• How Can Swelling Soil Be Recognized?
It is possible to visually identify swelling soil. Soil
containing swelling clay will be very sticky when
wet, and may display cracks or have a puffy
popcorn texture when dry.
However, it is also possible that there will be
deeper layers of swelling soil present beneath the
property. Drilling and trenching can help to
identify swelling soil beneath the ground.

7. What Can Homeowner Associations Do To
Reduce/Prevent Swelling Soil?
• Routine Inspection and Maintenance. This
extends to all of the systems that were designed to
protect the property from swelling soil damage,
including slabs, walls, subsurface and surface
drainage, slopes and landscaping.
• Structural Floor System Maintenance. Ventilation
systems, floor grates and barriers should be well
maintained and sub floor soil conditions should be
monitored periodically.
Swelling (Expansive) Soils:

8. • Proper Maintenance and Irrigation Practices. Best
practices include: maintaining adequate runoff
drainage slopes; cleaning gutters and
downspouts; ensuring that lawns and gardens are
not over watered; properly maintaining sprinkler
systems; removing trees shrubs and flowers
planted too close to foundations; sealing old
construction joints and cracks that develop over
time; inspecting concrete and walls; and repairing
cracks that are found as soon as possible.
• Subsurface Drainage Maintenance. If an area
drain is installed, the Association should be aware
of its location and should have the systems
maintained and cleaned out regularly.
Swelling (Expansive) Soils:

9. • Surface Drainage Maintenance. Roof gutters
should be inspected at least twice a year (spring
and fall). All debris should be cleaned out of the
metal gutters and checked for rust. Downspouts
should be checked for clogging, and sprinkler
systems should be maintained.
• Slope Maintenance. Positive slopes must be
maintained, especially over backfill areas. Settling
of concrete sidewalks may necessitate the
removal and replacement of these elements, if it
results in reverse drainage.
Swelling (Expansive) Soils:

10. Second: Dilation of soil:
Introduction:
• dilatancy is the observed tendency of a compacted
granular soil to dilate (expand in volume) as it is
sheared. This occurs because the grains in a
compacted state are interlocking and therefore do
not have the freedom to move around one
another. When stressed, a lever motion occurs
between neighboring grains, which produces a
bulk expansion of the material. On the other hand,
when a granular material starts in a very loose
state it may initially compact instead of dilating
under shear. It can be calculated by Mohr circle of
strain.

11. What is dilatancy?
• Dilatancy is the volume change observed in
granular materials when they are subjected to
shear deformations. This effect was first described
scientifically by Osborne Reynolds in
1885/1886 and is also known as Reynolds
dilatancy.
Second: Dilation of soil:

12. Dilation of soil:
• Unlike most other solid materials, the tendency of
a compacted granular material is to dilate (expand
in volume) as it is sheared. This occurs because
the grains in a compacted state are interlocking
and therefore do not have the freedom to move
around one another. When stressed,
a lever motion occurs between neighboring grains,
which produces a bulk expansion of the material.
On the other hand, when a granular material starts
in a very loose state it may
initially compact instead of dilating under shear. A
sample of a material is called dilative if its volume
increases with increasing shear and contractive if
the volume decreases with increasing shear.

13. • Dilatancy is a common feature of
the soils and sands. Its effect can be seen when
the wet sand around the foot of a person walking
on beach appears to dry up. The deformation
caused by the foot expands the sand under it and
the water in the sand moves to fill the new space
between the grains.
• Dilatancy is also studied by geotechnical
engineers, and is a part of the broader topic of soil
mechanics.
Dilation of soil:

14. • Phenomenology:
• The phenomenon of dilatancy can be observed in
a simple shear test on a sample of dense sand. In
the initial stage of deformation, the volumetric
strain decreases as the shear strain increases. But
as the stress approaches its peak value, the
volumetric strain starts to increase. After some
more shear, the soil sample has a larger volume
than when the test was started.
• The amount of dilation depends strongly on the
density of the soil. In general, the denser the soil
the greater the amount of volume expansion under
shear. It has also been observed that the angle of
internal friction decreases as the effective normal
stress is decreased.
Dilation of soil:

15. Why is dilatancy important?
• Because of dilatancy, the angle of friction
increases as the confinement increases until it
reaches a peak value. After the peak strength of
the soil is mobilized the angle of friction abruptly
decreases. As a result, geotechnical engineering
of slopes, footings, tunnels, and piles in such soils
have to consider the potential decrease in strength
after the soil strength reaches this peak value.
Dilation of soil:

16. T H A N K Y O U !